1. Field of the Invention
The present invention relates to an optical node processor provided to an optical network system using wavelength division multiplexing (WDM) technology, particularly relates to an optical node processor, an optical network system and its control method provided with an optical add/drop multiplexer (OADM) for adding/dropping an optical signal of an arbitrary wavelength to/from a WDM optical signal.
2. Description of Related Art
A demand for data communication rapidly increases by the population of the Internet. As a result, data communication requires mass storage. Then, to efficiently increase communication traffic, WDM technology is extensively used. Recently, the configuration of a ring and a mesh has been adopted as the configuration of a network to make the configuration of the network correspond to the modern advanced society. Then, in an optical node processor to be each node of the ring and the mesh, technique for adding/dropping the information of an optical signal is required. In an optical network system using WDM technology, for technique for adding/dropping an optical signal of a specific wavelength in a state of the optical signal, there is OADM technique.
FIG. 1 shows the configuration of an optical node processor using conventional type OADM technique disclosed in Japanese published unexamined patent application No. 2000-354006.
The optical node processor 1 is provided with an optical add/drop multiplexer (OADM) unit 2 and two optical amplifiers 3A and 3B arranged before and after the optical add/drop multiplexer.
A predetermined wavelength of a WDM optical signal input to the optical node processor 1 is added/dropped by OADM unit 2 after the WDM optical signals are collectively amplified by the optical amplifier 3A in an input part of the optical node processor 1. The WDM optical signal output from OADM unit 2 is collectively amplified by the optical amplifier 3B in an output part of the optical node processor 1 again and is sent to a transmission line. In a large-scale network of a long-distance transmission system, the optical node processor having such configuration has been used so far.
Recently, not only in a long-distance transmission system but in such a metropolitan small-scale network as in a city, a demand for an optical network system using WDM technology increases because of the increase of communication traffic. The metropolitan network is characterized differently from a general backbone network of a long-distance transmission system in that the distance of a transmission line between each optical node processor is short. Then, as for the metropolitan small-scale network, a lower-priced and more efficient network than conventional type networks is needed to be immediately configured. Accordingly, as for an optical node processor, a low-priced and efficient processor fitted to the metropolitan network is also demanded.
FIG. 2 shows an example of network configuration provided with an optical node processor in the case of tandem connection. In each optical node processor, an optical signal of a predetermined wavelength is added/dropped. However, in a metropolitan small-scale network, the distance of a transmission line between optical node processors is short and the loss of the transmission line may be small. In such a case, in configuration without an optical amplifier in an output part of an optical node processor, a WDM optical signal can be also normally transmitted, however, a new problem that automatic power reduction (APR) control in which safety is considered is disabled occurs. APR control means control that optical output from an optical node processor on the upstream side is shut down because of transmission line failure on the downstream side by the disconnection of the transmission line and others. Owing to this control, a transmission line and processor on the downstream side can be prevented from being damaged by a high output optical signal from a disconnected location. As described above, APR control is important control to guarantee a network.